The location where your fileserver choice will point to, if you install one (eg: Samba Server, Proftpd).

The location where you can upload and store your media content, for other applications to use (eg: Kodi, Emby, Plex etc).

Where is my user data?
For all software you install in dietpi-software, you can access your user data with /mnt/dietpi_userdata. Regardless of where the data is physically stored, a symlink will automatically be created for you if needed.
To check where the physical location is, you can run the following command readlink -f /mnt/dietpi_userdata